1. Field of Invention
This invention pertains to an implantable cardiac device with means for testing the impedance of its electrodes, and more specifically to a device of this type wherein the electrodes are tested using a high frequency test signal.
2. Description of the Prior Art
Implantable defibrillators are used to provide various types of therapy to a cardiac patient, including, for example defibrillation. These devices consist of a hermetic housing implanted into a patient, and two sets of leads terminating in corresponding electrodes. One set of leads extends from the housing to one or more cardiac chambers. The second set of leads extends from the housing to one or more defibrillator electrodes contacting the myocardia. The housing contains electronic circuitry for monitoring the condition of the patient's heart, usually through the cardiac electrodes. The same set may also be used to provide antibrady- and/or antitachycardia therapy. The second set of electrodes are used to provide defibrillation therapy. The housing further contains a battery which provides power for both signal processing and for the pacing pulses and defibrillation shocks. One such defibrillator is described in U.S. Pat. No. 5,330,504, incorporated herein by reference.
One important parameter for the operation of such a device is the defibrillator electrode impedance. This impedance is indicative of the positioning and integrity of the leads and electrodes. Electrode impedance is also related to the defibrillation threshold. Finally, since successful cardiac defibrillation depends on the amount of energy applied to the cardiac tissue by an electrical shock, and this energy or duration of the shock is also dependent on electrode impedance. Thus, it can be seen that knowing the electrode impedance is important both during implantation, and during normal defibrillator operation.
Typically, until now, electrode impedance in an implanted defibrillator was measured at low voltage and/or current levels to insure that any testing signals used to determine the electrode impedance has no effect on the patient's heart, as disclosed in U.S. Pat. No. 5,215,081, incorporated herein by reference. However, it is believed that such low level testing may not provide an accurate impedance measurement. An alternate testing method is to apply a high voltage shock to the electrodes. However, this shock is painful and uncomfortable to the patient, and potentially dangerous.